1. Field of the Invention
The present invention relates generally to a control system and method for compensating for "speed effect" in a stand of a tandem cold mill. More particularly, it relates to maintaining a constant roll force and a constant roll gap to produce a relatively higher percentage of "on gauge" material in that portion of the workpiece traveling through the stand during the acceleration and deceleration phases of the mill. The length of the workpiece is on gauge relative to the gauge of the remaining length of workpiece rolled in the other phases of the mill.
During threading and tailing out of a material in either strip or sheet form in a tandem cold mill, the stands of the mill are driven at a relatively low rate of speed, and the tension of the material of the workpiece between adjacent stands is regulated by the speeds of the stands. During the "full run of the mill," the stands are driven at relatively high rates of speed, where the material tension regulators are switched from regulating the interstand tension by controlling the speeds of the stands to regulating the interstand workpiece tension by controlling the roll gap of each stand. That is, strip tension is controlled at low speeds by controlling the speeds of the stands and at high speeds by controlling the roll gap.
In the transition from the threading stage to the "full run stage" the stands are accelerated, and, in the transition from the "full run stage" to the tailing out stage, the stands are decelerated. During these acceleration and deceleration phases, there occurs what is known in the industry as "speed effect." This "speed effect" causes the roll force in the stand to increase when the mill accelerates and to decrease when the mill decelerates. This naturally results in a change in the actual roll gap. The actual roll gap is a sum of the mill stretch and the apparent roll gap, which is held constant in stretch and the apparent roll gap, which is held constant in that the roll gap mechanism is fixed.
It has been theorized, but not proven, that this "speed effect" results from the fact that as the stand speed increases, oil tends to get into the bearing chocks thereby forcing the roll gap to close resulting in the material "going thin." Likewise, when the stand speed decreases, oil tends to leave the bearing chocks thereby forcing the roll gap to open, resulting in the material "going heavy." This "speed effect" has also come to be known as "oil film effect." Regardless of whether or not this "oil film effect" theory is correct the opening and closing of the roll gap is a reality in the speed transitions of the mill.
2. Description of the Prior Art
It is well-known in the industry that "on gauge" material is produced by maintaining a relatively constant roll gap where the actual roll gap is regulated by considering the apparent roll gap and the modulus of the mill stand. Several systems including the interstand tension regulators, the entry automatic gauge control, and the delivery automatic gauge control are employed in the present day tandem cold mills for controlling the gauge in the workpiece. Some examples are disclosed in U.S. Pat. Nos. 3,740,983; 3,765,203; 3,768,286; 3,848,443; 4,011,743; 4,016,735; and 4,286,447.
These prior roll gap control systems attempt to maintain a constant roll gap to produce an on gauge material in the threading phase, in the high speed full run phase, or in the tailing out phase of the mill, without providing roll gap control means or method for compensating for speed effect or oil film effect occurring in the acceleration or deceleration phases of the mill.